This invention relates to electrographic printing systems employing one or more stylus print electrodes and specifically to an electrographic printing system that accomplishes recording by means of dielectric breakdown across all or portion of the recording medium. This breakdown produces a cavity or aperture in the medium exposing a toner, pigment or contrasting medium producing a visible image. This phenomenon has also been referred to as spark discharge and electro-thermographic printing.
An example of such a printing system is facsimile printers which produce marks on a recording medium comprising three layers, a thick base layer, a black conductive layer and a thin opaque white layer. Electrical discharge through the thin opaque white layer with a bare needle stylus moved over the surface of the medium, removes the opaque white layer in an imagewise manner to expose the contrasting black layer. The writing quality obtained to date with this method of printing has not been of high quality. Removal of the opaque white layer by this type of stylus leaves an image of ragged appearance.
More recently, a printing system has been suggested that employs one or more stylus electrodes together with a recording medium and a backup or complement electrode in the form of a substantially solid, conductive pigment or toner. An example of such a printing system is disclosed in U.S. Pat. No. 4,224,601. The pigment electrode may be in the form of a roller or a belt that rotates or moves in cooperation with the recording medium. Electrical pulses applied to the electrodes cause a dielectric breakdown through the medium and the establishment of a direct current flow between the energized stylus electrode and the pigment electrode without intentionally creating an aperture in the recording medium. The energy produced from this electrode energization is effective on the solid conductive pigment medium. A minute portion of the pigment medium is thermally ablated or removed, and attracted, migrated, or transferred to the recording medium forming a visible image (dot or line). Fusion of the pigment portion to the recording medium occurs under the influence of the current flow and the plastic state of the ablated pigment medium. The dot or line image produced is formed on the surface of the recording medium opposite to the stylus electrodes.
This printing system has the advantage of forming a visible image during the "creation" of the image as contrasted to electrostatic printing systems wherein a latent image is first formed, followed by toning to form a visible image and, possibly, image fusing or fixing, as occurs in conventional xerographic printers and in electrostatic stylus printers of the type disclosed in U.S. Pat. No. 3,859,960. However, the disadvantage of this printing system is that the discharge or breakdown path formed through the recording medium to obtain migration of the pigment medium to the recording medium is difficult to control. The breakdown will initially occur via the path of least resistance through the recording medium in a region close to the stylus electrode discharge. This path may not be formed directly below the stylus electrode at the time of electrode energization, as the resistivity properties of the recording medium are not uniform. Also, the amount of incremental indexing of the recording medium may not be sufficient to establish an independent breakdown path from an immediately previously established and formed breakdown path. As a result, discharge will occur again in the previously established breakdown path since this is the path of least resistance. These events continuously occurring across the recording medium produce an image of poor quality and resolution and render this printing technology unacceptable for commercial exploitation.
The electrographic printing system of the present invention operates on the dielectric breakdown principal but intentionally produces a cavity or aperture in the recording medium to permit image formation on the stylus electrode side of the recording medium. With a combination dielectric coated recording medium and a dielectric encapsulated stylus electrode, improvement in image creation and resolution can be obtained that is acceptable for commercial exploitation. This is because the creation and establishment of breakdown paths through the recording medium can be more accurately controlled than possible with the previously mentioned printing system. As a background caveat, the phenomenon occurring in the electrographic printing system disclosed herein should not be confused with the printing phenomenon occurring in printing systems as disclosed in U.S. Pat. Nos. 3,355,743; 3,377,598; 3,550,153 and 3,751,159. In the first place, each of the disclosed systems involve an electrostatic discharge phenomenon wherein a transfer of the toner or pigment medium is accomplished by the establishment of an electric field via a displacement current. The pigment medium migrates to the recording medium under the influence of the electric field created during localized heat energization. The printing system disclosed herein involves the establishment of a dielectric breakdown across a portion of or all of the recording medium and a direct current flow during stylus energization.
Secondly, printing in each of the systems disclosed in these patents occurs on the surface of the recording medium opposite to the stylus electrodes. In the printing system disclosed herein, printing occurs on the same side of the recording medium as the stylus electrodes.